Hardware needed to run PBX?

I’d like to setup 2-4 physical phones and run a PBX like {Free,Open}PBX or Asterisk. However I’m unclear on what hardware requirements are needed for the PBX server. Almost all the documentation I’ve seen for {Free,Open}PBX and Asterisk have been around VoIP/SIP phones. However in my case I really need physical RJ-11 connectivity since the phones I have are old and I’d like to use the existing phone lines. I don’t know a ton about POTS/PSTN, but I remember that they still worked during power outages so I suspect I need more than just a PCIe 56K modem in order to build a PBX server.